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MCS symposium on “the precapillary sphincter”
MICROVASCULAR
RESEARCH
MCS
3,426-427 (1971
Symposium
on “The
Precapillary
Sphincter”
Introduction PAUL A. NICOLL
The degree of functional autonomy ascribed to the microvascular system is intimately connected to the conceptual idea of a precapillary sphincter and its method of action. If the microvasculature in any limited site regulates transcapillary exchange there in accord to strictly local needsthrough sometype ofinherentfeedbackmechanism, the major element controlled is the precapillary sphincter on the inflow side. This idea visualizes the microcirculation ascomposedof many interrelated but locally controlled units that withdraw blood as needed from the high pressure-low volume arterial supply and dischargeinto the low pressure-high volume venular system.Transcapillary exchanges between blood and interstitial fluid then occur under the influence of the usual chemical and physical forces of diffusion, filtration, and perhaps pinocytosis. The microcirculation then is autonomous in the sensethat strictly local feedback systemsacting through the control of the precapillary sphincters regulate flow and pressure in the exchange beds. That the efficiency of this operation is dependent on flow and pressureconditions within the associatedmacrocirculatory vesselsin no way impairs the concept of microvascular autonomy. At the other extreme, the exchangebedsmay be consideredto consistof passive,more or lessleaky, elements of the circulation. Although the samephysical forces regulate the exchangesbetween blood and interstitial fluid theseare now basically dependent on the functional statesof the arterial supply and venous drainage. Local factors, if regulatory, must act on the larger vesselsor on some characteristic of the exchange system itself, for example, capillary permeability. Larger anatomical units should reflect simultaneously similar exchange characteristics and central reflex mechanismswould tend to exert greater control of transcapillary activities. Many intermediate positions between these two extremes can readily be assumed. The very marked anatomical differences between the microvascular beds of the liver, muscle, skin, fat storage sites,etc., provide ample support for significant differences in functional controls of transcapillary exchange in these places. Another indeterminate is the wide variance as to what constitutes the microcirculation. Border conditions are very nearly as numerous as are the individuals working in the field. In addition, the term “precapillary sphincter” may include any constrictive activity along the terminal twigs of the arteriolar system that modify flow and pressurewith the exchange beds beyond. In the following papersthe authors consider the “precapillary sphincter” first from the anatomical basis, then its possible contribution to microcirculatory behavior, and in the final two contributions the nature of its regulation both by humeral and nervemediated paths. Although inclusions of investigations other than their own offer in 426
MCS
SYMPOSIUM
427
depth perspectives of each consideration, each author has approached the problem in his own individual manner, and considers the subject primarily based on his own research experience. Thus agreement, even in basic terms and concepts, should not be anticipated. It is hoped, however, that a clearer picture of what, where, when, and how “precapillary sphincters” exist and function will result from consideration of the data reviewed.
RESEARCH
MCS
3,426-427 (1971
Symposium
on “The
Precapillary
Sphincter”
Introduction PAUL A. NICOLL
The degree of functional autonomy ascribed to the microvascular system is intimately connected to the conceptual idea of a precapillary sphincter and its method of action. If the microvasculature in any limited site regulates transcapillary exchange there in accord to strictly local needsthrough sometype ofinherentfeedbackmechanism, the major element controlled is the precapillary sphincter on the inflow side. This idea visualizes the microcirculation ascomposedof many interrelated but locally controlled units that withdraw blood as needed from the high pressure-low volume arterial supply and dischargeinto the low pressure-high volume venular system.Transcapillary exchanges between blood and interstitial fluid then occur under the influence of the usual chemical and physical forces of diffusion, filtration, and perhaps pinocytosis. The microcirculation then is autonomous in the sensethat strictly local feedback systemsacting through the control of the precapillary sphincters regulate flow and pressure in the exchange beds. That the efficiency of this operation is dependent on flow and pressureconditions within the associatedmacrocirculatory vesselsin no way impairs the concept of microvascular autonomy. At the other extreme, the exchangebedsmay be consideredto consistof passive,more or lessleaky, elements of the circulation. Although the samephysical forces regulate the exchangesbetween blood and interstitial fluid theseare now basically dependent on the functional statesof the arterial supply and venous drainage. Local factors, if regulatory, must act on the larger vesselsor on some characteristic of the exchange system itself, for example, capillary permeability. Larger anatomical units should reflect simultaneously similar exchange characteristics and central reflex mechanismswould tend to exert greater control of transcapillary activities. Many intermediate positions between these two extremes can readily be assumed. The very marked anatomical differences between the microvascular beds of the liver, muscle, skin, fat storage sites,etc., provide ample support for significant differences in functional controls of transcapillary exchange in these places. Another indeterminate is the wide variance as to what constitutes the microcirculation. Border conditions are very nearly as numerous as are the individuals working in the field. In addition, the term “precapillary sphincter” may include any constrictive activity along the terminal twigs of the arteriolar system that modify flow and pressurewith the exchange beds beyond. In the following papersthe authors consider the “precapillary sphincter” first from the anatomical basis, then its possible contribution to microcirculatory behavior, and in the final two contributions the nature of its regulation both by humeral and nervemediated paths. Although inclusions of investigations other than their own offer in 426
MCS
SYMPOSIUM
427
depth perspectives of each consideration, each author has approached the problem in his own individual manner, and considers the subject primarily based on his own research experience. Thus agreement, even in basic terms and concepts, should not be anticipated. It is hoped, however, that a clearer picture of what, where, when, and how “precapillary sphincters” exist and function will result from consideration of the data reviewed.